Aircraft: B737
Phase of flight: Initial Approach
Anomaly: Inflight Event / Encounter Other / Unknown

On approach into BUR; we were assigned 3;000 MSL until VNY and cleared the visual to Runway 8. We were number three on the approach. During the approach; we heard both aircraft in front of us report TCAS RA's over the VNY airport. Both aircraft reported 'climb' RA's. Both aircraft reported that there was no traffic in sight and ATC reported no conflicting traffic in the area. There were aircraft at VNY; but well below a TCAS issue. As we continued towards the runway; we paid close attention to the TCAS display. The only traffic on our display was traffic 2;000 FT below us. As we approached VNY; about one mile prior; a TCAS target popped up with an immediate 'Descend' RA. At no time was there a TA that we can remember and the traffic showed as +1 and almost immediately on our nose. There would have been almost no possible way to not see something at that position. I; as the Pilot Flying; disconnected the autopilot to comply with the RA. We did not see any traffic visually and ATC reported no traffic other than the low traffic at VNY. Although we were pretty confident there was something in the area giving false RA's; we complied as long as we could as we were descending over an active airport. The RA continued for what seemed like a long time even though we were well past any traffic on the display. Due to the location of the event; where we ended up and the fact that the RA was a descend; I decided that a go-around was not prudent due to the terrain north; south; and east of the airport. We were already fully configured for the approach. Once everything went away; we were about 1;500 AGL and about one dot low on the glide path. I leveled off and made the decision to continue to land as that appeared to be the safest course of action based on traffic; our location; and the unknown of what was causing all the RA's for multiple aircraft with no apparent traffic. This whole situation was very strange due to the fact that it happened to multiple aircraft. As the First Officer and I discussed this after the flight; we both agreed that even though the procedure for a RA on approach was a go-around; this situation was somewhat unique and we took the safest action based on what we knew at the time. I think if we had received a 'Climb RA' like the other aircraft; a go-around would have been appropriate. After landing; I called Dispatch and informed them of what happened so they could inform other Burbank arrivals to be aware of the issue. On visual approach in VMC to Runway 8 at Burbank; we heard the preceding aircraft experience a RA and they initiated a go-around. Then they decided to continue the approach. The discussion with Tower ensued and they described what they had over Van Nuys; but no traffic was sighted by either the aircraft or Tower. Tower told us that the two preceding aircraft received RA's over Van Nuys; but he did not have any traffic to report. We continued the approach and crossed over Van Nuys at 3;000 FT and began down the glideslope. We were paying attention to the TCAS but saw no traffic. Once we started down; we got an RA directing a slight increased descent. I noticed on the TCAS; the traffic was on the First Officer side 200-300 FT lower than us. I looked outside; but could not see any traffic. The Captain increased the descent rate in an attempt to initially comply with the RA. The intruder aircraft now showed 300-500 FT lower than us as we continued our descent. The Captain decided to shallow the descent since we were slightly below the glide path at this time. The next time I noticed the intruder aircraft position on the TCAS; it had moved to 500 below us and was on the Captain's side of the aircraft. Then it disappeared and we never got a 'clear of conflict.' We then continued the visual approach and landed uneventfully. We relayed our experience to Tower. The following aircraft also experienced a TCAS Alert; but did not see any aircraft either. That was four aircraft with false TCAS Alerts on final that we heard of. B737 flight crew reports a TCAS RA at 3;000 FT over VNY during approach to BUR; but no aircraft is sighted. The two aircraft preceding the B737 and one following also reported TCAS RA's over VNY with no traffic sighted.